The mesosphere and the astenosphere are both part of the mantle. The mesosphere is the deeper part, while the astenosphere is the outer part of the mantle. Both of these layers have the same chemical composition, but the temperature and pressure are different in them. The mesosphere has higher temperature and pressure, so even though it is very hot, the rock is actually solid. The astenosphere has lower temperature and pressure, so the rock is more liquid.
